The invention relates to the attachment of electrical signal leads to portions of an electrical circuit. In particular, the invention is concerned with the attachment of leads to a circuit that operates at relatively high temperatures of about 900.degree. F. or higher, but is connected to equipment that operates at or near ambient temperature. A particular application is that of a solid electrolyte type of oxygen sensor. In one such type of device, a voltage which is generated between porous platinum electrodes affixed to opposite faces of a yttrium stabilized zirconia disc must be connected to an instrument amplifier. The disc and its electrodes typically operate at temperatures in excess of 900.degree. F. An example of such a sensor can be found in Habdas and Aaron U.S. Pat. No. 4,229,275. In said sensors, a previously fired stabilized zirconia disc is placed into an unfired ceramic tube and sealed thereto by shrinkage of the tube as it is fired at about 2400.degree. F. Platinum paste stripes are then painted along the inside surface of the tube and a glass frit contained within the paste securely bonds to the ceramic tube and the electrodes as the assembly is additionally fired at a temperature of about 1800.degree. F. Preferably, the ends of the stripes terminate in enlarged pads. If the ends of the sensor containing the pads can be maintained at temperatures below about 400.degree. F., connections to the pads can be made by ordinary solder connections or mechanical connectors. However, in many cases, low temperatures cannot be maintained and the temperature approaches the high operational temperature of the zirconia disc. In such a situation, mechanical connectors are difficult to use, and high temperature silver solders cannot be readily applied.
One attempted solution was to take a platinum lead wire of about 0.010" diameter and fire it into the platinum paste pad. The resulting bond was extremely good, but the lead wire was extremely weak from a mechanical standpoint and would break after several flexings at its point of connection. When the diameter was increased to about 0.020" to provide more mechanical strength, it was found to resist flexural breaking but would break away from the pad and ceramic when subjected to thermal shock stresses since the glass frit paste seems to more firmly adhere to the high expansion rate ceramic rather than to the platinum wire which expands less. Even when small and large diameter holes were drilled in the ceramic before firing and small and large diameter wires were shrunk-fit into them and bonded to the pads with platinum paste, the thinner wires broke when flexed and the paste broke away from the thicker wires when thermally cycled several times between room temperature and 900.degree. F.